Composite catalyst for hydrogenation of carbon dioxide and preparation method thereof

Abstract

The present disclosure relates to a composite catalyst hydrogenation of carbon dioxide to oxygenates, including higher alcohols. The composite catalyst comprises an in situ alkali metal- promoted cobalt-copper (CoCu) catalyst supported on zirconium dioxide (ZrO2), wherein the alkali metal is selected from the group consisting of potassium (K) and sodium (Na), forming the in situ alkali metal-promoted CoCuZr and CoCuFeZr composite catalyst herein denoted as CoCu / ZrO2-K or CoCu / ZrO2-Na. The present disclosure also relates to a method of preparing the composite catalyst and further relates to a catalytic method for hydrogenation of carbon dioxide to oxygenates, including higher alcohols.

Description

COMPOSITE CATALYST FOR HYDROGENATION OF CARBON DIOXIDE AND PREPARATION METHOD THEREOF CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of Singapore application no.10202403504X filed on 8 November 2024, the contents of it being hereby incorporated by reference in its entirety for all purposes.TECHNICAL FIELD

[0002] The present disclosure generally relates to a composite catalyst for the hydrogenation of carbon dioxide and a preparation method thereof. In particular, the present disclosure relates to a composite catalyst for the hydrogenation of carbon dioxide to produce oxygenates, including higher alcohols.BACKGROUND

[0003] The conversion of carbon dioxide into useful products offers a promising route for utilizing greenhouse gas emissions and supporting carbon neutralization. Research in carbon dioxide utilization (CU) has targeted a wide range of value-added products, including methane, long-chain hydrocarbons, olefins, methanol, and higher alcohols. Among these, higher alcohols (C2+alcohols) are important chemical building blocks with applications in the chemical, pharmaceutical, and energy sectors.

[0004] Conventionally, higher alcohols are produced via sugar fermentation or hydration of petroleum-derived alkenes. However, both processes face challenges such as low yields, long reaction times, limited substrate scope, and high energy demand. Direct carbon dioxide hydrogenation to higher alcohols is therefore an attractive and sustainable alternative. Nevertheless, this approach suffers from competing side reactions that generate carbon monoxide (CO), methane (CH4), light hydrocarbons (C2-C4), C1+acids and higher hydrocarbons (C5+), resulting in low selectivity and modest yields of C2+alcohols.

[0005] Several metals supported catalysts were reported for effective conversion of carbon dioxide to higher alcohols. For example, cobalt-based catalysts combined with metals such as Fe, Cu, and Mo and supported on various substrates have been widely studied for the conversion of carbon dioxide and syngas via Fischer–Tropsch synthesis to produce hydrocarbons. However, these systems exhibit poor water-gas shift (WGS) activity and tend to favor methanation, particularly when carbon dioxide is used as the feedstock instead of syngas. As a result, selectivity toward higher alcohols and oxygenates remains low, despite achieving satisfactory carbon dioxide conversion rates.

[0006] It is therefore desirable to provide a composite catalyst capable of achieving high carbon dioxide conversion with enhanced selectivity toward higher alcohols and oxygenates, to address at least one of the problems described hereinabove, or at least to provide an alternative.SUMMARY

[0007] In accordance with a first aspect of the present disclosure, a composite catalyst for hydrogenation of carbon dioxide to oxygenates, including higher alcohols is provided. The composite catalyst comprises an in situ alkali metal-promoted cobalt-copper (CoCu) catalyst supported on zirconium dioxide (ZrO2), wherein the alkali metal is selected from the group consisting of potassium (K) and sodium (Na), forming the in situ alkali metal-promoted CoCuZr composite catalyst herein denoted as CoCu / ZrO2-K or CoCu / ZrO2-Na.

[0008] In accordance with a second aspect of the present disclosure, a method of preparing an alkali metal -promoted composite catalyst for hydrogenation of carbon dioxide to oxygenates, including higher alcohols is provided. The method comprises mixing a precursor solution containing cobalt nitrate, copper nitrate and zirconium nitrate with a solution containing an alkali metal to obtain a reaction mixture; subjecting the reaction mixture to digestion to obtain a precipitated solid in the reaction mixture; separating the precipitated solid from the reaction mixture to obtain a separated precipitated solid, adjusting pH of the precipitated solid to be in the range of 7 to 9; drying the precipitated solid; calcinating the precipitated solid; and selectivewashing the precipitated solid to adjust concentration of the alkali metal in the calcined precipitated solid to obtain an in situ alkali metal-promoted CoCuZr composite catalyst; wherein the alkali metal is selected from the group consisting of potassium and sodium, and wherein the in situ alkali metal -promoted CoCuZr composite catalyst is herein denoted as CoCu / ZrO2-K or CoCu / ZrO2-Na.

[0009] In accordance with a third aspect of the present disclosure, a catalytic method for hydrogenation of carbon dioxide to oxygenates, including higher alcohols is provided. The catalytic method comprises converting a feed stream comprising carbon dioxide and hydrogen to oxygenates, including higher alcohols in the presence of a composite catalyst, wherein the composite catalyst comprises an in situ alkali metal-promoted cobalt-copper (CoCu) catalyst supported on zirconium dioxide (ZrO2), the alkali metal is selected from the group consisting of potassium (K) and sodium (Na), and forming the in situ alkali metal-promoted CoCuZr composite catalyst herein denoted as CoCu / ZrO2-K and CoCu / ZrO2-Na.

[0010] In some embodiments, the in situ alkali metal -promoted CoCuZr composite catalyst further comprises iron (Fe), herein denoted as CoCuFe / ZrC>2-K or CoCuFe / ZrCh-Na.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Various embodiments of the present disclosure are described hereinbelow in the detailed description with reference to the following drawings:FIG l is a diagram illustrating the distribution of products produced during carbon dioxide (CO2) hydrogenation to higher alcohols using CoCuZr composite catalysts with different concentrations of potassium (K).FIG 2 is a schematic diagram illustrating the synthesis of CoCuZr composite catalyst with in situ addition of alkali metal in accordance with an exemplary embodiment of the present disclosure.FIG. 3A is a bar chart showing the CO2 conversion rate using CoZr and CoCuZr catalysts, respectively with alkali metal such as potassium (K) or sodium (Na) as a promoter for CO2 hydrogenation to higher alcohols.FIG 3B is a bar chart showing the C-selectivity using the CoZr and CoCuZr catalysts, respectively with alkali metal such as potassium (K) or sodium (Na) as a promoter for CO2 hydrogenation to higher alcohols.FIG. 3C is a bar chart showing the selectivity and yield of oxygenate using the CoZr and CoCuZr catalysts, respectively with alkali metal such as potassium (K) or sodium (Na) as a promotor for CO2 hydrogenation to higher alcohols.FIG. 3D is a bar chart showing the selectivity of C2-C4 using the CoZr and CoCuZr catalysts, respectively with alkali metal such as potassium (K) or sodium (Na) as a promotor for CO2 hydrogenation to higher alcohols.FIG. 4A is a graph showing the time on stream activity for CO2 hydrogenation to higher alcohols using CoCuZr catalyst with high (or excess) amount of potassium, denoted as Co3Cu2 / ZrO2-KE.FIG. 4B is a graph showing the time on stream activity for CO2 hydrogenation to higher alcohols using CoCuZr catalyst with low amount of potassium, denoted as Co3Cu2 / ZrO2-KL.FIG. 5A shows the X-ray diffraction (XRD) patterns of the CoZr and the CoCuZr catalysts, in which potassium (K) was incorporated during synthesis at a concentration of about 5 wt.%, denoted as “K5”.FIG. SB shows the X-ray diffraction (XRD) patterns of the Co3Cu2Zr-K5 catalysts recovered before calcination (“Red-rec”), after calcination (“Cal-rec”), and after activation (“Act-rec”). FIG. 5C shows the thermogravimetric (TGA) and derivative thermogravimetric (DTG) curves of the Co3Cu2Zr5-K5 catalyst recovered after calcination (Co3Cu2Zr5-K5 Cal-rec).FIG. 5D shows the DTG-TGA curves of the Co3Cu2Zr5-K5 catalyst recovered after activation (Co3Cu2Zr5-K5 Act-rec).FIG. 6A shows a gas chromatography (GC) chromatogram of the products obtained from CO2 hydrogenation using the CoCuZr catalyst.FIG. 6B shows GC-MS results of the products obtained from CO2 hydrogenation using the CoCuZr catalyst.FIG. 6C shows the proton nuclear magnetic resonance (H-NMR) spectrum of the various CoCuZr catalysts.FIG. 7A shows the yields of oxygenates obtained using catalysts of various compositions under reaction conditions of 300 °C and 325 °C, at a pressure of 50 bar (5 MPa), with a gas flow (CO2 / H2) rate of 50 mL / min, a space velocity of 3 L / gcat / h, CO2 / H2 ratio of 1:3, and a reaction time of 24 hours.FIG. 7B shows the CO2 conversion and oxygenate selectivity using the [(Co3Cu2)10Fe-Zr-K5] catalyst, under reaction conditions of 300 °C and 350 °C, at a pressure of 50 bar (5 MPa), with a gas flow (CO2 / H2) rate of 50 mL / min, a space velocity of 3 L / gcat / h, CO2 / H2ratio of 1:3, and a reaction time of 24 hours.FIG. 7C shows the CO2 conversion and oxygenate selectivity using the [(Co3Cu2)10Fe-Zr-K5] catalyst, carried out in a single reactor, under reaction conditions of 325 °C, at a pressure of 50 bar (5 MPa), with a gas flow (CO2 / H2) rate of 50 mL / min, a space velocity of 3 L / gcat / h, CO2 / H2 ratio of 1:3, and a reaction time of 100 hours.DESCRIPTION

[0012] The following description sets forth exemplary methods, parameters, and the like. The embodiments are described in sufficient detail to enable those skilled in the art to practise the invention. Other embodiments may be utilized, and structural and logical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0013] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and / or combinations and / or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0014] In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0015] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance, e.g. within 10% of the specified value.

[0016] As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items.

[0017] By “comprising” it is meant including, but not limited to, whatever follows the word “comprising”. Thus, use of the term “comprising” indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present.

[0018] By “consisting of’ is meant including, and limited to, whatever follows the phrase “consisting of’. Thus, the phrase “consisting of’ indicates that the listed elements are required or mandatory, and that no other elements may be present.

[0019] The term “oxygenates” as used herein refers to organic compounds containing at least one oxygen atom in their molecular structure. Oxygenates may include, but are not limited to, alcohols, ethers, aldehydes, ketones, carboxylic acids, and esters

[0020] The term “higher alcohols” as used herein refers to alcohols containing two or more carbon atom (CV), and may include linear, branched, or cyclic structures. Higher alcohols include, but are not limited to, ethanol, propanol, butanol, pentanol, and their isomers, as well as longer-chain alcohols.

[0021] The present disclosure relates to a composite catalyst for hydrogenation of carbon dioxide (CO2) to produce oxygenates including higher alcohols. The composite catalyst comprises an in situ alkali metal-promoted cobalt-copper (CoCu) catalyst supported on zirconium dioxide (ZrO2), wherein the alkali metal is selected from the group consisting of potassium (K) and sodium (Na), forming the in situ alkali metal-promoted CoCuZr composite catalyst herein denoted as CoCu / ZrO2-K or CoCu / ZrO2-Na.

[0022] The terms “ CoCuZr” and “CoCu / ZrCh” as used herein refer to a composite catalyst comprising cobalt-copper (CoCu) catalyst supported on zirconium dioxide (ZrCh). The terms “CoCuZr” and “CoCu / ZrCh” may be used interchangeably without departing from the scope of the present disclosure.

[0023] The term “ alkali metal -promoted CoCuZr composite catalyst” as used herein refers to a composite catalyst comprising cobalt-copper (CoCu) catalyst supported on zirconium dioxide, modified by an alkali metal promoter. The alkali metal promoter is selected from potassium (K) or sodium (Na). The “-K” or “-Na” added after “ CoCu / ZrO?”, i.e. CoCu / ZrO2-K or CoCu / ZrO2-Na refers to CoCuZr composite catalyst modified by a potassium (K) or a sodium (Na) promoter, respectively.

[0024] The composite catalyst may alternatively be represented as Co3Cu2 / ZrO2 or Co2Cu3 / ZrO2. In some embodiments, the composite catalyst may be represented as Co3Cu2 / ZrO2-K or Co2Cu3 / ZrO2-K for composite catalyst containing potassium (K), or represented as Co3Cu2 / ZrO2-Na or Co2Cu3 / ZrO2-Na for composite catalyst containing sodium (Na).

[0025] Tn various embodiments, the alkali metal -promoted CoCuZr composite catalyst is prepared by a direct co-precipitation method. In this method, the concentration of the alkali metal promoter may be adjusted to desired amount by modifying the selective washing protocol to obtain a solid after complete precipitation. The alkali metal employed in the method not onlyserves as a precipitating agent, it also serves as an in situ source for alkali metal incorporation into the solid CoCuZr composite catalyst.

[0026] In various embodiments, the alkali metal in the composite catalyst has a concentration ranging from 0.1 to 15 wt.%, based on the total weight of the composite catalyst. In some embodiments, the alkali metal in the composite catalyst has a concentration ranging from 5 to 15 wt.%, based on the total weight of the composite catalyst. In certain embodiments, the alkali metal in the composite catalyst has a concentration ranging from 0.1 to 5 wt.%, based on the total weight of the composite catalyst. The amount of alkali metal present in the composite catalyst determines the distribution of products produced by a CO2 hydrogenation process.

[0027] For example, referring to FIG. 1, when the composite catalyst comprises a relatively high (or excess) concentration of potassium, denoted as CoCuZr-KE in FIG. 1, the catalyst may possess high selectivity toward a mixture of Ci+acids and C2+alcohols. In this embodiment, the concentration of potassium in the composite catalyst may range from 5 to 15 wt.%, based on the total weight of the composite catalyst. When the composite catalyst comprises a relatively low concentration of potassium, denoted as CoCuZr -KL in FIG. 1, the catalyst may exhibit high selectivity toward C? alcohols and their corresponding isomers. In this embodiment, the concentration of potassium in the composite catalyst may range from 0.1 to 5 wt.%, based on the total weight of the composite catalyst. This clearly indicates that the high potassium coverage on the catalyst surface leads to low hydrogen adsorption phenomenon, which has been well explored for light olefins selectivity against light alkanes production for CO2 hydrogenation process.

[0028] In various embodiments, the composite catalyst may have a pH ranging from 7 to 9. In some embodiments, the composite catalyst has a pH ranging from 7 to 8 This pH range may be applied when low concentration of alkali metal is employed. In some embodiments, thecomposite catalyst has a pH of 9. This pH may be applied when high concentration of alkali metal is employed.

[0029] In various embodiments, the weight ratio of cobalt (Co) to copper (Cu) to zirconium (Zr) to potassium may range from 20:30:45:5 to 30:20:45:5. Tn some embodiments, the weight ratio of Co to Cu to ZrtoNa may range from 30:20:45:5 to 10:40:45:5. In certain embodiments, the weight ratio of Co to Cu to Zr to Na may range from 30:20:45:5 to 10:50:35:5. In yet other embodiments, the weight ratio of Co to Cu to Zr to Na may range from 30:20:45:5 to 50:20:25:5.

[0030] In some embodiments, the cobalt (Co) may be present in a concentration ranging from 1 to 70 wt.%, based on the total weight of the composite catalyst.

[0031] In some embodiments, the copper (Cu) may be present in a concentration ranging from 1 to 70 wt.%, based on the total weight of the composite catalyst.

[0032] In some embodiments, the zirconium (Zr) may be present in a concentration of 29 wt.%, based on the total weight of the composite catalyst.

[0033] In various embodiments, the alkali metal-promoted CoCuZr composite catalyst may further comprise iron (Fe), herein denoted as CoCuFe / ZrO2-K or CoCuFe / ZrO2-Na. In some embodiments, the composite catalyst containing Fe may be represented as [(Co3Cu2)10Fe / ZrO2], In the embodiments where the composite catalyst comprises potassium (K) or sodium (Na), the composite catalyst may be represented as (Co3Cu2)10Fe / ZrO2-K or (Co3Cu2)10Fe / ZrO2-Na. The composite catalysts are capable of achieving an yield of about 15 wt.% of oxygenates. In some embodiments, the weight ratio of higher alcohol to organic acid in the oxygenates is 60:40.

[0034] In some embodiments, the amounts of (Co3Cu2), Fe, Zr, and K present in the composite catalyst are 10 to 40 wt.%, 5 to 20 wt.%, 10 to 50 wt.% and 0.1 to 10 wt.%, respectively, based on the total weight of the composite catalyst.

[0035] According to a second aspect of the present disclosure, a method of preparing an alkali metal -promoted composite catalyst is provided. Generally, the method involves a co-precipitation process comprising preparing a precursor solution followed by digestion, precipitation, pH adjustment, drying, calcination, and selective washing to obtain an alkali metal-promoted composite catalyst for hydrogenation of carbon dioxide to oxygenates including higher alcohols.

[0036] Referring to FIG. 2, the method comprises mixing a precursor solution 201 containing cobalt nitrate, copper nitrate and zirconium nitrate with a solution containing an alkali metal 202 to obtain a reaction mixture; subjecting the reaction mixture to digestion 203 to obtain a precipitated solid in the reaction mixture; separating 204 the precipitated solid from the reaction mixture to obtain a separated precipitated solid; adjusting pH of the precipitated solid to be in the range of 7 to 9; drying 205 the precipitated solid; calcinating 206 the precipitated solid; and selective washing (not shown) the precipitated solid to adjust concentration of the alkali metal in the precipitated solid to obtain an in situ alkali metal-promoted CoCuZr composite catalyst. In various embodiments, the alkali metal is selected from the group consisting of potassium and sodium, forming the in situ alkali metal-promoted CoCuZr composite catalyst herein denoted as CoCu / ZrCh-K and CoCu / ZrCh-Na.

[0037] In various embodiments, the pH of the precipitated solid may be adjusted to a range of 7 to 9. This may be achieved by employing a water-washing protocol, in which the precipitated solid is repeatedly washed with distilled water until the pH is maintained at 7, 8 or 9 as desired.

[0038] In some embodiments, the pH of the precipitated solid may be adjusted to a range of 7 to 8, which corresponds to low concentration of alkali metal in the precipitated solid. The low concentration of alkali metal may range from 0.1 to 5 wt.%, based on the total weight of the composite catalyst. In some embodiments, the precipitated solid may be adjusted to have a pH of 9, which corresponds to high concentration of alkali metal in the precipitated solid. The high concentration of alkali metal may range from 5 to 15 wt.%, based on the total weight of the composite catalyst. In some embodiments, the concentration of the alkali metal in theprecipitated solid may be adjusted to 0.1 to 15 wt.%, based on the total weight of the composite catalyst.

[0039] In various embodiments, the selective washing of the precipitated solid to adjust the concentration of the alkali metal may be carried out after the precipitated solid has undergone calcination 206 or further calcination 207. In some embodiments, the selective washing of the precipitated solid may be carried out after the solid filtration or separation 204, before the precipitated solid is subjected to one or more distinct washing approaches, depending on the desired chemical characteristics, before calcination and drying.

[0040] In general, the selective washing may be carried out using a Neutral Washing protocol or a Limited Washing protocol.

[0041] The Neutral Washing protocol is carried out to remove residual alkali and neutralize the solid sample. The procedure generally involves thoroughly washing the precipitated solid obtained from the preceding process with deionized water. The pH of the resulting filtrate is monitored using a pH meter, and the washing process continues until the pH stabilizes at around 7, indicating that the precipitated solid has been neutralised and excess alkali has been removed.

[0042] The Limited Washing protocol is carried out to retain in situ generated alkali metals that may be beneficial for subsequent processing or material properties. The procedure involves washing the precipitated solid with a limited amount of deionized water while monitoring the pH to ensure it remains around 9. This controlled washing maintains a mildly alkaline environment and prevents or minimises leaching of alkali metals from the precipitated solid.

[0043] In various embodiments, the step of digestion may be performed at a temperature ranging from 25 to 80 °C, preferably at room temperature, for a period of 6 to 12 h. The resulting precipitate is then separated from the reaction mixture. Any suitable method of separation may be employed. Such methods include, but are not limited to, filtration, centrifugation, sieving, the like or combinations thereof. In some embodiments, separation is carried by filtration.

[0044] In various embodiments, the precipitated solid is calcined at a temperature ranging from 400 to 550 °C, for a period of 6 to12 h to get the oxide form of the respective metal composites such as Co, Cu, Fe, K as K₂O and ZrO₂ oxides. In some embodiments, the precipitated solid is calcined at about 450 °C for 6 h.

[0045] In some embodiments, the calcined precipitated solid may be subjected to further calcination. The calcined precipitated solid may be further calcined at a temperature ranging from 300 to 550 °C, for a period of 6 to 12 h. In some embodiments, the precipitated solid is calcined at about 300 °C for 6 h.

[0046] Unless otherwise specified, any suitable methods may be employed for operations such as mixing, separating or drying, without departing from the scope of the present disclosure.

[0047] In some embodiments, the method may further include adding an iron (III) nitrate to the precursor solution to form the in situ alkali metal-promoted CoCuZr composite catalyst, herein denoted as CoCuFe / ZrO₂-K or CoCuFe / ZrO₂-Na.

[0048] In various embodiments, weight ratio of cobalt nitrate, copper nitrate, iron (III) nitrate and zirconium nitrate used for forming the catalyst precursor solution may be selected based on the desired metal composition, target metal concentration, and the atomic weight of each metal element. The relative proportions of the respective nitrate may therefore vary to achieve the intended molar or atomic ratios of the final composite catalyst. Accordingly, the amounts of the starting compounds are not fixed, but may be adjusted to form the desired composite catalyst. In some embodiments, the precursor solution is prepared by combining a metal precursor solution having a concentration of about 1 M with a base solution having a concentration ranging from 0.5 to 2 M.

[0049] In various embodiments, the solution containing the alkali metal comprises an aqueous solution of potassium or sodium salts selected from hydroxides, carbonates, or acetates. In some embodiments, the solution containing the alkali metal comprises potassium carbonate or sodium carbonate. The solution containing the alkali metal provides the promoterspecies during the co-precipitation process and also participates in the in situ formation of the composite catalyst.

[0050] According to a third aspect, a catalytic method for hydrogenation of carbon dioxide to oxygenates, including higher alcohols is provided. The catalytic method comprises converting a feed stream comprising carbon dioxide and hydrogen to oxygenates, including higher alcohols in the presence of a composite catalyst, wherein the composite catalyst comprises an in situ alkali metal-promoted cobalt-copper (CoCu) catalyst supported on zirconium dioxide (ZrCh), wherein the alkali metal is selected from the group consisting of potassium (K) and sodium (Na), and forming the in situ alkali metal -promoted CoCuZr composite catalyst herein denoted as CoCu / ZrO₂-K and CoCu / ZrO₂-Na.

[0051] In various embodiments, the feed stream comprises carbon dioxide and hydrogen in a volume ratio ranging from 1:2 to 1:4, preferably about 1:3. In some embodiments, the feed stream may further include one or more inert gases such as nitrogen or argon, or carbon monoxide as a co-reactant

[0052] In some embodiments, the higher alcohols comprises C₁⁺ alcohols. The oxygenates may further include C₂⁺ isomerised hydrocarbon products and C₁⁺ acids. Other gases, such as carbon monoxide and methane may also be produced during the catalytic hydrogenation of carbon dioxide.

[0053] In some embodiments, the alkali metal in the composite catalyst has a high concentration ranging from 5 to 15 wt.%, for producing C₂⁺ higher alcohols, C₁⁺ acids or a combination thereof during the catalytic hydrogenation of carbon dioxide. In some embodiments, the alkali metal in the composite catalyst has a low concentration ranging from 0.1 to 5 wt.%, for producing predominantly C₂⁺ higher alcohols, and smaller amounts of C₂⁺ isomers, C₁⁺ acids, or a combination thereof. The high alkali metal concentration shows low CO2 conversion of less than 30%, whereby carbon monoxide (CO) is predominantly produced, while low alkali metal concentration in the composite catalyst improves CO2 conversion butfavours methane formation. Both the high and low alkali metal incorporated composite catalysts show 10 to 25 wt.% selectivity toward oxygenates. The results highlight the key findings that elucidate the role of the alkali metal, acting both as a promoter and a precipitating agent, as well as the influence of oxide catalysts and the effect of supports (ZrO₂) in the challenging reaction of CO₂ hydrogenation for the complex synthesis of higher alcohols.

[0054] In some embodiments, more than 60% of the feed stream may be converted to oxygenates including higher alcohols, gaseous products and hydrocarbons. In some embodiments, more than 60% of the feed stream may be converted, with a selectivity toward oxygenates ranging from 10 to 25 wt.%. In some embodiments, the oxygenates include higher alcohols with a yield of more than 6%.

[0055] In some embodiments, the composite catalyst may further include an iron (Fe), herein denoted as CoCuFe / ZrO₂-K or CoCuFe / ZrO₂-Na. In these embodiments, more than 60% of the feed stream may be converted, with a selectivity toward oxygenates of more than 15 wt.%.

[0056] The in situ alkali metal-promoted composite catalyst of the present disclosure demonstrates efficient CO2 hydrogenation, achieving more than 60% conversion with an impressive 10 to 25% selectivity toward oxygenates, including higher alcohols (C2+alcohols) and their corresponding isomers (C2+isomers), at 10 to 15% selectivity based on the total selectivity of the oxygenates. The formation of large amounts of oxygenates at high CO2 conversion represents an attractive feature, as it enables potential scale-up and the design of large-scale heterogeneous catalysis process. This catalytic technology described in the present disclosure supports global sustainability initiatives related to green energy and carbon capture and utilization (CCU). In short, the in situ alkali metal, and the use of oxide catalysts provide high CO2 conversion, high oxygenate yield, and a controllable effect of alkali metal concentration on product distribution.

[0057] The alkali metal plays a crucial role in inducing structural changes in the catalysts. A high alkali metal coverage on the catalyst surface results in a reduced hydrogen adsorptioncapacity, as evidenced by changes in product selectivity. Specifically, a mixture of Ci+acids were obtained for catalysts containing a high potassium concentration, in contrast to catalysts with a lower potassium concentration, which favoured the formation of higher alcohols. These findings strongly support ongoing efforts to advance CO2 hydrogenation toward higher alcohol production.

[0058] Overall, the catalyst exhibits efficient conversion performance without the need for extensive reduction. The in situ alkali metal and mild reduction using substances such as carbon dioxide and hydrogen in the feed stream did not cause a significant decrease in CO2 conversion but effectively improved selectivity toward the desired C2+alcohols and C2+isomers. These results clearly indicate that a combination of different oxide phases contributes to enhanced catalytic activity. Also, the alkali metal concentration has a pronounced influence on the design and development of suitable catalyst materials with product-oriented selectivity. The present disclosure provides valuable insights for identifying pathways in CO2 hydrogenation to complex higher alcohols.

[0059] The method of the present disclosure with the use of the composite catalyst yields products including higher alcohols (C2+alcohols), their corresponding isomers and light alcohols (C2-C4 alcohols). These alcohols serve as valuable intermediates in various industrial applications, including use as solvents, alternative fuels, fuel additives, and raw materials for daily chemical products. Conventionally, such alcohols are predominantly produced from petroleum-based feedstocks through energy -intensive processes. The present disclosure offers an alternative, sustainable route to these products via CO2 hydrogenation, thereby contributing to carbon utilization and reduction of fossil-based dependence.

[0060] To facilitate a better understanding of the present disclosure, the following examples of specific embodiments are given In no way should the following examples be read to limit or define the entire scope of the disclosure. One skilled in the art will recognize that the examples set out below are not an exhaustive list of the embodiments of this disclosure.EXAMPLESExample 1

[0061] Preparation of Composite Catalyst

[0062] The in situ alkali metal -pro oted CoCuZr composite catalysts were prepared by coprecipitation method.

[0063] In this example as illustrated in FIG. 2, the method comprises mixing aqueous solutions of metal salts such as cobalt nitrate, copper nitrate, and zirconium nitrate under stirring to obtain a precursor solution 201. A solution containing an alkali metal 202, such as potassium carbonate (K2CO3) was added to the precursor solution to form a reaction mixture. The reaction mixture was then subjected to a digestion step 203 at a temperature ranging from 25 to 80 °C, preferably at room temperature, for 6 h to promote crystallization to obtain a precipitated solid. Thereafter, the precipitated solid was separated by filtration 204, and pH of the precipitated solid was adjusted to 7 to 9. This was followed by drying 205 the precipitated solid at 100 °C (373 K) for 8 h before subj ecting the precipitated solid to calcination 206. The precipitated solid was calcined at a temperature of 400 °C (673 K) for 6 h, followed by activation 207 at 300 °C (573 K) for 12 h to obtain the oxide form of the composite catalyst. The oxide form of the composite catalyst was then subjected to selective washing protocol (not shown) to adjust the concentration of the alkali metal in the composite catalyst accordingly, to a high value or a low value, to obtain the in situ alkali metal -promoted CoCuZr composite catalyst.Example 2

[0064] Various compositions of the composite catalysts were prepared and evaluated, and the details thereof are shown in the tables below.

[0065] Table 1: Activity testing results of the various compositions of the composite catalysts.* The subscript number in the formula is the % ratio of the various components - e.g. CosnZr^Ks- ratio of Co: Zr: K is 50:45:5

[0066] The table below details the possible range of the components, the various promoters and the metal oxides used in preparing the composite catalysts.

[0067] Table 2: The detailed catalyst compositionsi“ Possible composition of catalysts that can he synthesized.Example 3

[0068] High-throughput tests were carried out to evaluate the performance of the composite catalysts under a range of conditions.

[0069] In this example, activity testing was conducted. Five (5) oxide catalysts comprising CoZr, and CoCuZr modified with potassium (K) and sodium (Na) were evaluated for catalytic performance in CO2 hydrogenation to higher alcohols. The 5 oxide catalysts were: Co₅ / ZrO₂-KE, Co₃Cu₂ / ZrO₂-KE, Co₂Cu₃ / ZrO₂-KE, Co₃Cu₂ / ZrO₂-KL and Co₃Cu₂ / ZrO₂-NaL, where “KE” denotes excess potassium, “KL” denotes low potassium, and “NaL” denotes low sodium. The results are shown in FIGs. 3A to 3D. The reactions were conducted at a space velocity of 3 L / gcat / h.

[0070] FIG. 3A shows the CO2 conversion rate using CoZr and CoCuZr as catalysts for carbon dioxide hydrogenation to higher alcohols. The CoCuZr catalysts containing high (excess) potassium (K) concentrations, denoted as Co₃Cu₂ / ZrO₂-KE and Co₂Cu₃ / ZrO₂-KE, exhibited lower CO2 conversion, at less than 30%, which is approximately two-times lower than that observed for CoCuZr catalysts containing low alkali metal concentrations, denoted as Co₃Cu₂ / ZrO₂-KL and Co₃Cu₂ / ZrO₂-NaL in FIG. 3A. The CoZr catalyst, denoted as Co₅ / ZrO₂-KE also exhibited high CO₂ conversion, at approximately 60%.

[0071] FIG. 3B shows the C-selectivity using the various CoZr and CoCuZr catalysts. The results show that CoCuZr catalysts with high potassium concentrations (KE) favour high CO selectivity, of more than 50%, whereas catalysts with low alkali metal concentrations (KL, NaL) result predominantly in CH4 formation, of more than 50%. Both the low and high alkali metalconcentration catalysts demonstrate high selectivity towards C2+alcohols and / or oxygenate (oxy) compounds. CoCuZr catalysts modified with low alkali metals exhibit high selectivity to higher alcohols, achieving C₂⁺ alcohol (C₂OH⁺) yields of more than 6% (FIG. 3C). Additionally, sodium-modified CoCuZr catalyst also show efficient activity, achieving more than 60% CO2 conversion (FIG. 3A), with high C2+alcohols or isomers formation.

[0072] The CoCuZr oxide catalysts of the present disclosure are able to achieve an efficient CO2 conversion of more than 60%, with more than 6% yield of oxygenates, which include C₂⁺ alcohols, C₂⁺ isomers, and C₁⁺ acids (FIGs. 3B and 3C). Reducing the space velocity from 6 L / gcat / h to 3 L / gcat / h exhibits high CO₂ conversion, and C2+alcohols (C2-OH) and C2+isomers selectivity.

[0073] The alkali metals not only play the role of change in CO2 conversion but also show difference in product distribution as shown in FIG. 1. The tests show that the CoCuZr catalyst containing high potassium (K) (CoCuZr-KE) possesses high selectivity towards C₁⁺ acids and C₁⁺ alcohols, while the CoCuZr catalyst containing low potassium (K) (CoCuZr-KL) does not show any prominent C₁⁺ acids peaks except for C₁⁺ alcohols, and C₂⁺ isomers, which have been evidenced based on primary observation from gas-chromatography-mass spectrometry (GC-MS) (FIG. 6B) and proton nuclear magnetic resonance spectroscopy (H₂-NMR) (FIG. 6C) tests. This clearly indicates that the high potassium coverage on catalyst surface leads to low hydrogen adsorption phenomenon, which has been well explored for light olefins selectivity against light alkanes production for CO2 hydrogenation.Example 4

[0074] The time on stream activity was evaluated for CO2 hydrogenation to higher alcohols using two different catalysts, namely, Co₃Cu₂ / ZrO₂-KE and Co₃Cu₂ / ZrO₂-KE.

[0075] The CO2 hydrogenation was carried out by heating the feed stream at a temperature of 300 °C for about 100 hours in the presence of the catalyst. The reaction was carried out at apressure of 50 bar (5 MPa), with a gas flow (CO2 / H2) rate of 50 ml / min, and a space velocity of 6 L / gcat / h. The volume ratio of CO2 to H2 in the feed stream is 1:3. The low and high potassium (K) concentration catalysts possess different activity profiles.

[0076] FIG. 4A is a graph showing the stream activity for CO₂ hydrogenation to higher alcohols using the high potassium (K) concentration Co₃Cu₂ / ZrO₂-KE catalyst, while FIG. 4B shows the stream activity using the low potassium (K) concentration Co3Cu2 / ZrC>2-KL catalyst. FIG. 4A shows that the Co₃Cu₂ / ZrO₂-KE catalyst achieved a stable conversion of more than 15% with a steady increase in CO selectivity of up to 60%. On the other hand, the Co₃Cu₂ / ZrO₂-KL catalyst achieved a high conversion of more than 22% at a constant space velocity of 6 L / gcat / h with methane (CH4) as the predominant product. The low K-containing catalyst possesses high selectivity toward C1-C3 alcohols, along with oxygenates. The cumulative selectivity toward C2+higher alcohols and C2+isomers is about more than 25%. The catalysts show stable performance after 24 hours, where the selectivity toward C1-C3 alcohols increased from 30 hours onwards with stable performance. In the overall activity comparison for single reactor and high-throughput tests, the trend in CO2 conversion and product distribution are very well matched.Example 5

[0077] Characterization of Catalysts

[0078] The catalysts, namely Co₃Cu₂ / ZrO₂-K5, Co₂Cu₃ / ZrO₂-K5 and Co₅Zr-K5 were analyzed by XRD and DTG-TGA to determine their crystalline phases and weight-loss behaviour after the reaction (FIGs. 5A to 5D). The term “K5” as used herein refers to the amount of potassium (5 wt.%) employed while synthesising the composite catalyst.

[0079] The results obtained for the CoCuZr catalysts with high potassium (K) concentration indicate that all calcined CoCuZr catalysts exhibited indexed crystalline phases CuO, Co-oxide and ZrO2-tetragonal phases. The three catalysts from the CoCu family recovered from the reaction formed different phases, particularly, the Co and Cu metallic phases, while theactivated CoCuZr catalysts (not calcined) showed mixture of Co and Cu metallic phases together with Co carbides phases.

[0080] DTG-TGA analysis of the catalysts recovered after the reaction was conducted to examine their weight-loss behaviour It was observed that only the activated catalysts (FIG 5D) showed significant carbon loss at temperatures above 500 °C, suggesting that the released carbon species likely originated from metallic carbide phases rather than from carbon deposition on the catalyst surface. The oxide catalyst did not show notable high-temperature carbon loss, which may be attributed to the loss of water molecules, followed by minor carbon losses associated with layered carbon in metal carbides on metal oxide or product carbon deposits on the catalyst surface.

[0081] The H₂-NMR tests carried out on the Co₃Cu₂ / ZrO₂-K5-Act and Co₃Cu₂ / ZrO₂-K5-Cal catalysts are shown in FIG. 6C.Example 6

[0082] Several composite catalysts comprising an in situ alkali metal-promoted cobaltcopper-iron (CoCuFe) catalyst supported on zirconium dioxide (ZrO2) were prepared in a manner similar to that described in Example 1, except that an iron (III) nitrate was also added to the precursor solution. The compositions of the composite catalysts prepared in this example are shown in Table 3.

[0083] Table 3: Various compositions of the composite catalysts prepared in this example

[0084] High-throughput tests were carried out on the samples prepared to evaluate the performance of the composite catalysts under a range of conditions.

[0085] The concentrations of the alkali metal in the CoCuFe / ZrCh composite catalyst were adjusted accordingly to high and low levels by employing the modified selective washing protocol of to obtain solids after complete precipitation, where alkali metals were used as precipitating agents and in situ sources for alkali metals in the solid composite catalysts. To check facile conversion and selectivity trend for carbon dioxide hydrogenation to oxygenates including higher alcohols, an iron-modified CoCu catalyst was synthesized using oxalate and co-precipitation method, followed by the addition of the alkali metal.

[0086] Among the 36 catalyst formulations with different metal and promoter combinations and techniques tested for carbon dioxide hydrogenation, Co, CoCu, and CoCuFe supported catalysts have shown high CO₂ conversion along with a 15% yield of oxygenate (FIG. 7A). The ratio of higher alcohols to organic acids in the oxygenates is 60:40.

[0087] Although CO2 hydrogenation to higher alcohols was well explored using CoCu, it requires pre-reduction and pre-activation steps to form metallic Co and Co- carbide phases, where there was no such report found in the literature so far on oxide CoCu catalysts which has shown efficient activity to higher alcohols. The well-developed oxide CoCuFe catalyst system shows an efficient CO2 conversion of more than 60%, with an yield of more than 15% of oxygenates (C2+alcohols, Cl+acids) (FIG. 7B). The same reaction conditions as the one described earlier in this example were employed, except for the reaction temperature, which ranges from 300 to 350 °C. The composite catalyst used in this example is denoted as [(Co3Cu2)10Fe-Zr-K5], The increasing selectivity to oxygenates by the addition of iron (Fe) along with the CoCuZr catalyst system suppresses methane (CH4) formation.

[0088] The detailed catalyst characterization evidenced the responsible phases, and species were helpful for the enhancement of catalytic activity along with the stability of catalysts. Among five screened lOOh stability test catalysts, CoCuFe prepared by co-precipitation method showed satisfactory CO2 conversion of more than 60%, and 25 to 28 wt.% oxygenate selectivity (FIG. 7C). It showed more than 100 hours stability without losing performance. Reproducibility tests were also conducted, and the results showed a similar trend in catalytic activity performance when compared with single-reactor testing. For the single-reactor activity tests that were carried out, the same reaction conditions as the one described earlier in this example were employed, except that the reaction was carried out for 100 hours as compared to 24 hours. Consistent CO2 conversion with oxygenate selectivity were observed. The composite catalyst used in this example is denoted as [(Co3Cu2)10Fe-Zr-K5]-single reactor.Example 7

[0089] The catalytic CO2 hydrogenation reactions were carried out under the conditions summarized in the table below.

[0090] Table 4: A summary of the reaction conditions used for evaluating the composite catalysts of the present disclosure.

[0091] As shown in Table 4, in some embodiments, the catalytic CO2 hydrogenationreactions may be carried out in a fixed-bed reactor under the conditions summarized in Table 4. In an exemplary embodiment, a mixture of carbon dioxide and hydrogen gases may be introduced at a CO₂:H₂ volume ratio of approximately 1:3. The reaction may be conducted at temperatures ranging from 300 °C to 325 °C, and under a pressure of 50 bar (5.0 MPa), depending on the specific catalyst composition. The gas hourly space velocity (GHSV) may be maintained at 3000 mL / gcat / h. The reaction effluent may be analyzed by gas chromatography to determine conversion and product selectivity toward oxygenates, including higher alcohols and their corresponding isomers.

[0092] Although embodiments of the invention have been shown and described, the invention is not limited to the described embodiments. Instead, it would be appreciated by those skilled in the art that various modifications and variations can be made to the embodiments of the invention without departing from the scope of the invention, the scoop of which is set forth in the following claims.

Claims

CLAIMS:

1. A composite catalyst for hydrogenation of carbon dioxide to oxygenates, including higher alcohols, the composite catalyst comprising an in situ alkali metal-promoted cobaltcopper (CoCu) catalyst supported on zirconium dioxide (ZrO2), wherein the alkali metal is selected from the group consisting of potassium (K) and sodium (Na), forming the in situ alkali metal-promoted CoCuZr composite catalyst herein denoted as CoCu / ZrCL-K or CoCu / ZrCh-Na.

2. The composite catalyst of claim 1, wherein the alkali metal in the composite catalyst has a concentration ranging from 0.1 to 15 wt.% based on the total weight of the composite catalyst.

3. The composite catalyst of claim 1, wherein the alkali metal in the composite catalyst has a high concentration ranging from 5 to 15 wt.%, based on the total weight of the composite catalyst.

4. The composite catalyst of claim 1, wherein the alkali metal in the composite catalyst has a low concentration ranging from 0.1 to 5 wt.%, based on the total weight of the composite catalyst.

5. The composite catalyst of claim 3, wherein the composite catalyst has a pH of 9.

6. The composite catalyst of claim 4, wherein the composite catalyst has a pH ranging from 7 to 8.

7. The composite catalyst of claim 1, wherein the weight ratio of Co, Cu, Zr and K ranges from 20:30:45:5 to 30:20:45:5.

8. The composite catalyst of claim 1, wherein the weight ratio of Co, Cu, Zr and Na ranges from 30:20:45:5 to 10:40:45:5.

9. The composite catalyst of claim 1, wherein the cobalt (Co) is present in a concentration ranging from 1 to 70 wt.%, based on the total weight of the composite catalyst.

10. The composite catalyst of claim 1, wherein the copper (Cu) is present in a concentration ranging from 1 to 70 wt.%, based on the total weight of the composite catalyst.

11. The composite catalyst of claim 1, wherein the zirconium (Zr) is present in a concentration of 29 wt.%, based on the total weight of the composite catalyst.

12. The composite catalyst of any one of claims 1 to 4, wherein the alkali metal-promoted CoCuZr composite catalyst further comprises an iron (Fe), herein denoted as CoCuFe / ZrO₂-K or CoCuFe / ZrO₂-Na.

13. A method of preparing an alkali metal -promoted composite catalyst for hydrogenation of carbon dioxide to oxygenates, including higher alcohols, the method comprising:mixing a precursor solution containing cobalt nitrate, copper nitrate and zirconium nitrate with a solution containing an alkali metal to obtain a reaction mixture;subjecting the reaction mixture to digestion to obtain a precipitated solid in the reaction mixture;separating the precipitated solid from the reaction mixture to obtain a separated precipitated solid;adjusting pH of the precipitated solid to be in the range of 7 to 9;drying the precipitated solid;calcinating the precipitated solid; andselective washing the precipitated solid to adjust concentration of the alkali metal in the precipitated solid to obtain an in situ alkali metal-promoted CoCuZr composite catalyst; wherein the alkali metal is selected from the group consisting of potassium and sodium, and wherein the in situ alkali metal -promoted CoCuZr composite catalyst is herein denoted as CoCu / ZrO2-K or CoCu / ZrO2-Na.

14. The method of claim 13, wherein the concentration of the alkali metal in the precipitated solid is adjusted to 0.1 to 15 wt.%, based on the total weight of the composite catalyst.

15. The method of claim 13, wherein the alkali metal -promoted CoCuZr composite catalyst has a high concentration of alkali metal ranging from 5 to 15 wt.%, with a pH of 9.

16. The method of claim 13, wherein the alkali metal -promoted CoCuZr composite catalyst has a low concentration of alkali metal ranging from 0.1 to 5 wt.%, with a pH of 7 to 8.

17. The method of claim 13, further comprising:adding an iron (111) nitrate to the precursor solution to form the in situ alkali metal-promoted CoCuZr composite catalyst, herein denoted as CoCuFe / ZrC -K or CoCuFe / ZrCh-Na.

18. The method of claim 13, wherein the step of digestion is performed at a temperature ranging from 25 to 80 °C, for a duration of 6 to 12 h.

19. The method of claim 13, wherein the step of selective washing is performed before the step of adjusting the pH of the precipitated solid.

20. A catalytic method for hydrogenation of carbon dioxide to oxygenates, including higher alcohols, the catalytic method comprising:converting a feed stream comprising carbon dioxide and hydrogen to oxygenates, including higher alcohols in the presence of a composite catalyst,wherein the composite catalyst comprises an in situ alkali metal -promoted cobaltcopper (CoCu) catalyst supported on zirconium dioxide (ZrCb), the alkali metal is selected from the group consisting of potassium (K) and sodium (Na), and forming the in situ alkali metal-promoted CoCuZr composite catalyst herein denoted as CoCu / ZrCh-K and CoCu / ZrCh-Na.

21. The catalytic method of claim 20, wherein the oxygenates further comprises C₂⁺ isomerised hydrocarbon products and C₁⁺ acids.

22. The catalytic method of claim 20, wherein the alkali metal in the composite catalyst has a high concentration ranging from 5 to 15 wt.%, for producing C₂⁺ higher alcohols, C₁⁺ acids or a combination thereof.

23. The catalytic method of claim 20, wherein the alkali metal in the composite catalyst has a low concentration ranging from 0.1 to 5 wt.%, for producing C2+higher alcohols, C isomers, Cr acids or a combination thereof24. The catalytic method of claim 20, wherein more than 60% of the feed stream is converted, with a selectivity toward oxygenates ranging from 10 to 25 wt.%.

25. The catalytic method of claim 24, wherein the oxygenates include C2 higher alcohols with a yield of more than 6%.

26. The catalytic method of claim 20, wherein the composite catalyst further comprises an iron, herein denoted as CoCuFe / ZrCh-K or CoCuFe / ZrOz-Na.

27. The catalytic method of claim 26, wherein more than 60% of the feed stream is converted, with a selectivity toward oxygenates of more than 15 wt.%.

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